Improving the fuel efficiency of automobiles has recently become an important issue from the viewpoint of global environment conservation. Active efforts have been made to reduce the weight of car bodies themselves by increasing the strength of materials used and reducing the thickness of members. While various kinds of hot-rolled steel sheets heretofore used for automobile parts have a grade in terms of tensile strength of 440 MPa or 540 MPa, there has recently been an increasing need for high strength hot-rolled steel sheets having a grade of 780 MPa or higher. On the other hand, increasing the strength of steel sheets is generally accompanied by a decrease in formability. Thus, various studies have been carried out with regard to improvements in stretch flangeability (or hole expandability) required for steel sheets to be used as automobile parts.
For example, Japanese Unexamined Patent Application Publication No. 2006-274318 describes a method for manufacturing high strength hot-rolled steel sheets which includes hot rolling a steel slab containing C at 0.05 to 0.15%, Si at not more than 1.50%, Mn at 0.5 to 2.5%, P at not more than 0.035%, S at not more than 0.01%, Al at 0.02 to 0.15% and Ti at 0.05 to 0.2% at a finishing temperature of not less than the Ar3 transformation point, thereafter cooling the steel sheet to the temperature range of 400 to 550° C. at a cooling rate of not less than 30° C./s followed by coiling, and cooling the coiled coil to not more than 300° C. at an average cooling rate of 50 to 400° C./h, thereby manufacturing a hot-rolled steel sheet which has a microstructure containing bainite at 60 to 95% by volume as well as ferrite or ferrite and martensite. The technique of JP '318 is described as being capable of manufacturing high strength hot-rolled steel sheets with excellent hole expansion workability which have a sheet thickness of about 2 mm and exhibit a tensile strength of not less than 780 MPa and a hole expanding ratio of not less than 60%.
Further, Japanese Unexamined Patent Application Publication No. 4-329848 describes a high strength hot-rolled steel sheet which has a composition containing C at 0.03 to 0.25%, Si at not more than 2.0%, Mn at not more than 2.0%, P at not more than 0.1%, S at not more than 0.007%, Al at not more than 0.07% and Cr at not more than 1.0% and satisfying {(Si+20P)/(Mn+Cr)}=0.6 to 1.5, and is formed of ferrite and a second phase wherein the hardness of the second phase Hv is 200 to 600, the volume fraction of the second phase is 5 to 40%, the grain diameter of the second phase is not more than 25 μm, and a specific relationship is satisfied by the sum of the product of the hardness and the volume fraction of ferrite plus the product of the hardness and the volume fraction of the second phase. It is described that the technique of JP '848 can produce high strength hot-rolled steel sheets with excellent fatigue strength and stretch flangeability which exhibit a strength exceeding 490 MPa.
Japanese Unexamined Patent Application Publication No. 2009-280900 describes a method for manufacturing high strength hot-rolled steel sheets having a tensile strength of not less than 780 MPa which includes hot rolling a steel slab containing C at 0.04 to 0.15%, Si at 0.05 to 1.5%, Mn at 0.5 to 2.0%, P at not more than 0.06%, S at not more than 0.005%, Al at not more than 0.10% and Ti at 0.05 to 0.20% at a finishing temperature of 800 to 1000° C., thereafter cooling the steel sheet at a cooling rate of not less than 55° C./s and subsequently at a cooling rate of not less than 120° C./s for the temperature range of not more than 500° C. so as to cool the steel sheet by nucleate boiling cooling, and coiling the steel sheet at 350 to 500° C. It is described that according to the technique of JP '900, a high strength hot-rolled steel sheet with a tensile strength of not less than 780 MPa is obtained which has a microstructure containing more than 95% of bainite and less than 5% of inevitable other phases and exhibits excellent stretch flangeability after working as well as stably small variations in quality inside the steel sheet.
Further, Japanese Unexamined Patent Application Publication No. 2000-109951 describes a method for manufacturing high strength hot-rolled steel sheets having excellent stretch flangeability which includes heating a steel slab containing C at 0.05 to 0.30%, Si at not more than 1.0%, Mn at 1.5 to 3.5%, P at not more than 0.02%, S at not more than 0.005%, Al at not more than 0.150% and N at not more than 0.0200% and further containing one or two of Nb at 0.003 to 0.20% and Ti at 0.005 to 0.20% to a temperature of not more than 1200° C., hot rolling the steel slab at a finish roll-starting temperature of 950 to 1050° C. and a finish roll-finishing temperature of not less than 800° C., initiating cooling within 2 seconds after the completion of the rolling and continuously cooling the steel sheet to a coiling temperature at an average cooling rate of 20 to 150° C./s, and coiling the steel sheet at 300 to 550° C. A hot-rolled steel sheet manufactured by the technique of JP '951 is described to exhibit high strength with a tensile strength of not less than 780 MPa and to exhibit excellent stretch flangeability because it has a microstructure based on fine bainite having an average grain diameter of not more than 3.0 μm and is free from mixed grains or coarse grains with a grain diameter exceeding 10 μm.
Japanese Unexamined Patent Application Publication No. 2000-282175 describes a method for manufacturing ultrahigh strength hot-rolled steel sheets having excellent workability which includes casting a steel slab containing C at 0.05 to 0.20%, Si at 0.05 to 0.50%, Mn at 1.0 to 3.5%, P at not more than 0.05%, S at not more than 0.01%, Nb at 0.005 to 0.30%, Ti at 0.001 to 0.100%, Cr at 0.01 to 1.0% and Al at not more than 0.1% and satisfying 0.05≦(% Si+% P)/(% Cr+% Ti+% Nb+% Mn)≦0.5, immediately thereafter or after once cooling the steel slab heating the steel slab to 1100 to 1300° C. and hot rolling it at a finish roll-finishing temperature of 950 to 800° C., initiating cooling within 0.5 seconds after the completion of the rolling and cooling the steel sheet at a cooling rate of not less than 30° C./s, and coiling the steel sheet at 500 to 300° C. A hot-rolled steel sheet manufactured by the technique of JP '175 is described to exhibit high strength with a tensile strength of not less than 980 MPa and to have a microstructure which includes bainite as a main phase at a volume fraction of not less than 60% and less than 90% and at least one of pearlite, ferrite, retained austenite and martensite as a second phase and in which the bainite phase has an average grain diameter of less than 4 μm. The steel sheet is also described to exhibit excellent workability.
According to the technique described in JP '318, stretch flangeability is improved by increasing toughness, namely by lowering the fracture appearance transition temperature by means of reducing the segregation of phosphorus in ferrite grain boundaries. However, the technique of JP '318 has a problem in that it is extremely difficult to improve stretch flangeability if the steel does not contain ferrite or the ferrite content is extremely low. Further, the technique described in JP '848 has a problem in that because the fraction of the soft ferrite phase is 60% or more, the steel sheet cannot stably ensure high strength meeting the recent need for as high a strength as 780 MPa or more; namely, the strength of the steel sheet is insufficient. Further, while the technique described in JP '900 can ensure high strength with a tensile strength of not less than 780 MPa, the steel sheet does not still have sufficient fatigue resistance required for automobile parts because controlling of the bainite phase microstructure is insufficient.
The technique described in JP '951 provides a very fine bainite microstructure. However, because niobium and titanium remain without being dissolved during heating of the slab, sufficient amounts of dissolved titanium and niobium cannot be ensured, thus resulting in insufficient fatigue resistance in some cases. According to the technique described in JP '175, phases other than the bainite phase are present at least in excess of 10% and thus the homogenization of the microstructure is insufficient, resulting in insufficient stretch flangeability in some cases.
It could therefore be helpful to provide a high strength hot-rolled steel sheet having high strength with a tensile strength of not less than 780 MPa as well as exhibiting excellent stretch flangeability and excellent fatigue resistance.